A basic object of a finder assembly is the formation of the image of an object with an objective optical system and then observation of the formed object image with an eyepiece optical system. To reduce the weight and cost of the finder assembly, various attempts have been made recently to replace part or all of the image composing lenses by resins or plastics. However, the shape and refractive index of a plastic lens varies greatly in response to temperature or humidity changes.
Recently, a Kepler-type finder having an objective optical system with a positive power has been designed such that lenses having a strong power are used in a composing lens group to reduce the overall size. Since lenses having strong power vary greatly in optical performance due to changes in their shape or refractive index, the position of the object image formed by the objective optical system will become unfocussed greatly from the plane of the field frame.
A finder assembly usually has a mechanism for adjusting the diopter setting, which has heretofore been adapted to adjust the diopter setting of the eyepiece optical system. Such a conventional mechanism for adjusting the diopter setting may be capable of ensuring that the focus of the eyepiece optical system is brought into registry (e.g., aligned) with the position in which the image is formed by the objective optical system. However, in the conventional mechanism, the position of the image formed by the objective optical system and the position of the field frame is changed so much that the field frame is blurred such that the field frame and the object image cannot be observed clearly simultaneously. A further problem with the large change in the position of the image formed by the objective optical system is that the mechanism for adjusting the diopter setting of the eyepiece optical system is insufficient to permit the object image per se to be observed clearly through effective adjustment of the diopter setting.